Lasers are commonly used in a wide variety of research and development applications including spectroscopy, biotechnology applications, and industrial operations such as inspecting, processing, and micromachining a variety of media and substrates. In many of these applications, a pulsed laser may be used at a random, pseudo-random, and/or non-constant pulse repetition frequency (“PRF”).
Certain laser applications, such as film trimming processes on silicon wafers, use overlapped laser pulses to make cuts on thin resistance film to change its resistance value to be within a desired accuracy range. Such a process may employ laser pulses at different PRFs and different overlapping, while the laser pulses should be substantially equal in their pulse energy, pulse width, and pulse peak power for high trimming quality.
In typical prior art lasers, a lasing medium may be pumped using an optical pump source. However, the laser energy per pulse may decrease with increasing PRF (e.g., due to reduced pumping time between pulses), while laser pulse width may increase with increasing PRF (e.g., due to reduced pumping time that results in lower lasing gain in the lasing medium). These issues may be particularly pronounced in Q-switched solid state lasers.
As discussed above, many laser applications use laser pulses at different PRFs. It may be desirable for some applications to maintain substantially constant pulse energy and pulse width at different PRFs. For example, in thin film trimming on silicon, inadequate laser pulse energy may result in incomplete trimming, while too much laser energy may result in unacceptable damage to the passivation structure or integrated circuit substrate.
Various approaches have been taken to ensure that laser operation remains within an acceptable process window (e.g., within defined pulse parameters for peak power, pulse energy, pulse width, and other parameters). For example, U.S. Pat. No. 4,337,442 titled “FIRST LASER PULSE AMPLITUDE MODULATION,” which is assigned to the assignee of the present application, describes a method of laser pulse amplitude control by controlling laser pumping current.
U.S. Pat. No. 6,947,454, titled, “LASER PULSE PICKING EMPLOYING CONTROLLED AOM LOADING,” issued to Sun et al., which is assigned to the assignee of the present application, describes a method for providing stable laser pulses at random intervals by blocking unused laser pulses with a pulse picking device, such as an acousto-optic modulator (AOM), while keeping the laser operating at a constant PRF.
U.S. Pat. No. 5,226,051 titled, “LASER PUMP CONTROL FOR OUTPUT POWER STABILIZATION,” attempts to equalize pulse energy by providing current via pumping diodes according to a simple “step-type” function. In this approach, the lasing medium may be pumped at a first constant pumping level during a first pumping period, and at a second, lower constant level following the first pumping period during a second pumping period. This technique may work in laser applications where the PRF is relatively low. However, because it uses only two constant pumping currents, pulse equalization is less satisfactory. As such, this type of system may not be capable of delivering desired pulse equalization, nor capable of operating at higher PRFs.
FIG. 1 illustrates timing graphs of a prior art laser pumping system including a trigger signal 101, a pumping current signal 120 supplied by a current source, a graph 140 corresponding to stored energy in a lasing medium, and a graph 160 representing laser output pulses 162, 164.
The trigger signal 101 is used to initiate the Q-switch with the laser resonator for the generation of the laser pulses 162, 164. In order to produce the Q-switched laser pulses 162, 164, the lasing medium is energized by an optical pump source driven by a current or power source. The pump source may include, for example, a laser diode, diode bar or diode bar stack, or other pump source known in the art. The laser medium may include a solid state laser medium including, but not limited to neodymium-doped yttrium aluminum garnet (Nd:YAG), neodyminium-doped yttrium lithium fluoride (Nd:YLF), neodyminium-doped yttrium vandate (Nd:YVO4), or other solid state lasing mediums used in the art.
The trigger signal 101 may include square wave triggers 102, 104 to initiate the action of the Q-switch and generation of the laser pulses 162, 164 by the leading (falling) edges of the 102, 104. A pump controller may respond to the trigger signals 102, 104 to cause the current or power driven pump source to pump the lasing medium according to a step function as represented by the substantially square pumping current signal 120.
The pumping current signal 120 may be supplied to the pump source at a standard pumping level lS for a pumping period tr. The standard pumping level lS may be determined based on the PRF, pulse energy level, and pulse width used by the laser application. After the pumping time period tr, the pumping current 120 supplied to the pump source may be abruptly switched to a reduced, maintaining pumping level lN. The maintaining pumping level lN may be chosen, in some embodiments, to maintain the stored energy in the lasing medium at a desired or equalized level (e.g., equalized energy to produce an equalized laser pulse). Both the standard pumping level lS and the maintaining pumping level lN are substantially constant or flat.
The graph 140 shows an amount of stored energy in the lasing medium as a function of time with respect to the pumping current signal 120 and the trigger signal 101. During the pumping time period tr, the energy stored in the lasing medium increases as the lasing medium is pumped using the pumping current signal 120 at the standard level lS. This increase is shown at section 142 of graph 140. After the pumping time period tr, the pumping current signal 120 is abruptly reduced to the maintaining level lN, which causes the energy stored in the lasing medium to plateau at an energy level 144. The energy stored in the lasing medium is discharged (as indicated at reference 146) when the Q-switch allows a laser pulse to be emitted in response to the trigger signal 101. The energy level 144 may be selected such that the resulting laser pulse has acceptable power and pulse width according to the particular laser processing application. The Q-switches used may be an electro-optic Q-switch or an acousto-optic Q-switch, depending on the application and laser design.
The graph 160 shows the emission of the laser pulses 162, 164 relative to the trigger 101, the pumping current signal 120, and the graph 140 representing stored energy. At respective times corresponding to the laser pulses 162, 164, the Q-switch allows the lasing medium to emit laser pulse energy. As shown in the graph 140 representing the stored energy, this causes the energy stored in the lasing medium to be discharged from the lasing medium as a laser pulse 162, 164. Following the emission of a laser pulse 162, 164, the lasing medium may be re-energized by pumping the lasing medium at the standard pumping level lS for another pumping time period tr, and then at the maintaining pumping level rN.
For laser pulse firing after the pumping time period tr, the stored energy may be at an equalized level, therefore the laser pulses will be equalized (e.g., as long as the PRF is lower than that of 1/tr). If the laser is fired at time intervals less than that of the pumping time period tr, the lasing medium may not have been sufficiently energized to the equalized energy level when the laser pulse is emitted. This may result in a laser pulse with substantially less pulse energy and/or longer laser pulse width than intended. Prior art lasers may not be capable of delivering equalized laser pulses at high PRF because the substantially constant nature of the standard pumping level lS results in long pumping time periods tr used for the lasing medium to accumulate the desired stored energy. The maintaining pumping level lN is also of a substantially constant value. Due to the various details of different laser designs, materials used, and manufacturing processes, the pulse equalization effects within the desired PRF range may be unsatisfactory.